The present invention relates to devices that measure the volume of fluids, liquids, or pulverous solids or the distance from a predetermined point to a target. More particularly, the present invention relates to a sensor that provides accurate measurements of volume or distance in near-field conditions.
Fuel, lubricants, bulk solids, and a variety of other materials are stored in tanks and similar containers and then consumed by being drawn from the container as needed. In virtually all storage applications, the level (or volume) of the material in the storage container is monitored to ensure that the supply of material does not unexpectedly run out. Measurement sticks, electro-mechanical sensors, ultrasonic sensors, and other devices are used to monitor material levels.
While known level monitoring devices are functional, they do not provide satisfactory accuracy and information regarding the amount of material in a storage tank. Measurement sticks are undesirable because they require human manipulation. The measurement stick must be manually inserted into the material and then the markings on the stick must be read to determine an indication of the amount of material in the container. Measurement readings are subject to human error. Worse, if no person is available to take a reading, the amount of material in the tank cannot be determined.
Electro-mechanical level sensors (xe2x80x9ce-m sensorsxe2x80x9d) function automatically and don""t require human intervention. Nevertheless, these devices suffer from several deficiencies. First, e-m sensors must be calibrated to the size of the tank in which they are installed. Second, e-m sensors provide only one type of information, a measurement of the percentage of material remaining in the tank: such as the common xe2x80x9cF,xe2x80x9d xe2x80x9cxc2xe,xe2x80x9d xe2x80x9cxc2xd,xe2x80x9d xe2x80x9cxc2xc,xe2x80x9d and xe2x80x9cExe2x80x9d level designations. Third, e-m sensors use mechanical floats. Float devices often register inaccurate readings due to changes in tank orientation, which occur when storage tanks are mounted in vehicles. Lastly, e-m sensors are unreliable due to failures in their moving parts.
Ultrasonic sensors don""t rely on mechanical floats. Instead, ultrasonic sensors measure an echo signal reflected off the surface of the material in the storage tank. Nevertheless, ultrasonic sensors are affected by a variety of environmental variables, such as temperature, target location, target composition and motion, transmission media, and acoustic noise. Most known ultrasonic sensors fail to adequately compensate for changes in one or more of these variables. In particular, most ultrasonic level sensors are unable to accurately measure material levels when the surface of the material is close to the ultrasonic transducer in the sensor.
A conventional ultrasonic level sensor 10 is shown in FIG. 1. The sensor 10 is controlled by a driver (not shown). The driver generates electrical signals that are delivered to a transducer (not shown), within the sensor. The transducer is a resonant piezoelectric element that vibrates in response to the electrical signals from the driver. The oscillation of the piezoelectric element creates a sound wave 12 that propagates from the sensor 10 to a target 14. When the sound wave 12 reaches the target 14, at least a portion of it is reflected back toward the sensor 10 as an echo signal 16.
The sensor 10 measures the amount of time required for the sound wave 12 to travel to the target 14 and the echo signal 16 to travel from the target 14 back to the sensor 10. The time needed for the sound to make this round trip is referred to as the time of flight (xe2x80x9cTOFxe2x80x9d), and may be used by a microprocessor (not shown) coupled to the sensor to calculate the distance of the target from the sensor. If the sensor is mounted on a container, the distance measurement may be used to calculate a volumetric representation of the amount of material in a container.
The transducer continues to vibrate for a certain period of time after the signals from the driver 30 are removed or reduced to zero magnitude. This time is referred to as a xe2x80x9cring time.xe2x80x9d The ring time of an ultrasonic sensor such as the sensor 10 is dependent on numerous variables including temperature, humidity, the magnitude of the drive signal applied, and the type of crystal used in the transducer. Generally, rings times range from a few microseconds to 2000 xcexcsecs.
When an ultrasonic sensor, such as the sensor 10, attempts to measure the level of a material whose surface is very close to the transducer, the echo signal returns to the transducer before the end of the transducer ring time. The sensor does not detect an echo that occurs while the transducer is still ringing. The undetected echo signal, however, reflects off of the sensor and back to the target a second time and sometimes multiple times. By the time multiple reflections occur, the transducer stops ringing. The transducer then detects a second, third, or consecutive reflection and mistakes it for the first reflection. This situation is shown in FIG. 2. A drive pulse DP causes the transducer to ring for a certain ring time which is detected as a low signal on a detect signal line. An echo pulse EC received during the ring time is not differentiated on the detect signal line from the low signal produced by the ringing oscillator. Thus, no echo detection occurs until after the oscillator has stopped ringing.
Detection of an echo reflection rather than the first echo causes a sensor to produce erroneous measurements. In the worst-case scenario, the time of flight measurement error approaches twice the ring time of the transducer. For a sensor with a ring time of 500 xcexcsecs and a full-scale range of one meter the error is:
Error=2xc3x97(speed of soundxc3x97ring time)/2=346 m/sxc3x97500e-6 meters=0.173 meters or 17.3%
An error of this magnitude is unacceptably large for many applications. Accordingly, a number of approaches have been developed to address the problem. One approach to the problem is to use two transducers: one for receiving the sound pulse and one for transmitting the sound pulse. Measurement systems of this type are complex and expensive due to the additional hardware used. Another approach is to use a physical spacer to prevent the sensing of close range targets. Measurement systems of this type have limited usefulness. Yet another approach to the problem is to reduce the ring time of the oscillator with a clamp or dampener. However, a dampened transducer is not as sensitive as a non-dampened transducer. A device can include a spacer and a dampened oscillator, but this is also unsatisfactory.
Accordingly, there is a need for an improved sensor that provides accurate and enhanced material distance and level information. In particular, there is a need for a sensor that provides accurate material distance and level information in near field conditions (i.e., situations where the surface of the material is physically proximate to the transducer in the sensor).
The present invention provides a near field measurement sensor. The sensor includes a transducer such as an ultrasonic transducer. The transducer is coupled to a controller such as a programmable microprocessor or microcontroller. The controller generates a first command signal for the transducer, detects a first echo signal from the transducer, and determines whether the transducer received the first echo signal within a near-field time. If the first echo signal was received within the near-field time, the controller detects a second echo signal of a predetermined magnitude. If a second echo signal is not detected within a predetermined amount of time, the controller ignores the first echo signal and generates a second command signal different than the first command signal. The controller modifies the second command signal until a second echo signal is detected. The controller converts either the first echo signal or the difference between the first echo signal and the second echo signal to a distance representation to the target or a volumetric representation of the amount of material in the container. Preferably, the transducer is driven at resonance with a series of pulses such that the frequency of the pulses matches the transducer""s temperature-dependent frequency characteristics.
To ensure that the controller can distinguish between a true echo signal rather than a secondary echo reflection, the controller is programmed to measure the ring time of the oscillator in the transducer by monitoring the output of the transducer continuously following a trigger command. The controller considers the ring time complete once the detect signal returns high for a predefined period of time. A temperature sensor is coupled to the controller and the controller is programmed to compensate for changes in temperature that can affect ring time and other measurement variables. The controller determines the ring time of the transducer before making each measurement. The near field time is set at about two and one-third times the ring time.
The controller may utilize a look up table to generate a command signal that is a series of pulses tuned to the transducer""s resonant frequency. The look up table includes information that is based on temperature. Accordingly, the system includes a temperature sensor to provide temperature information to the controller. Information from the temperature sensor may be distributed to other devices and other locations through a communications module coupled to the controller.
The invention may also be implemented as a method of sensing the level of material in a container, where the ambient temperature of the sensor""s surrounding is detected. The method involves generating a signal having a frequency and an amplitude. The signal is sent to a target or surface of the material of interest, a first reflection of the signal from the target is detected according to a timing scheme, and the amplitude and frequency of the signal are controlled according to the detected temperature and the time of detection of the reflection of the signal.
In the case of near field measurements, the method also includes detecting a second reflection, determining the time difference between the first reflection and the second reflection, and converting the time difference between the first reflection and the second reflection to a distance representation or a volumetric representation of the amount of material in the container.
Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings.